Add devel/ for prototype files

This adds user/devel for prototype of new features. Currently there
are 2 files.

version.c - versioned pointer for writable snapshots
shard.c   - update optimized directory index based on hash

TODO: we may want to compile those files (so add files to Makefile, or
add new devel/Makefile)

Signed-off-by: Daniel Phillips <daniel@tux3.org>
Signed-off-by: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>